Turbine blades rotate about a central axis of a gas turbine engine. Blades may rotate within the compressor portion of the turbine or the "hot" combustion portion of the turbine. An engine case (the "seal") shrouds the turbine blades. The tips of the turbine blades and seal are maintained in very close relation because the efficiency of the turbine engine is inversely related to the leakage of gases between the turbine blades and the seal.
While rotating at high speeds, the tips of the turbine blades often contact the seal. Such contact can result in wear damage to the blades from abrasion caused by shear forces on the blades. Wear damage to the tips of the blades results in decreased engine efficiency and expensive replacement costs. To minimize damage to the turbine blade tips from contact with the seal, the blade tips may be either made from materials harder than the seal or coated with a material harder than the seal. An effectively "harder" blade tip avoids wear damage to itself by instead abrading the seal.
It is known in the art of turbine blade coatings that the desirable properties of a coating are dependent on a variety of factors, including chemical composition, method of application, coating density, existence of cracks (microcracks and macrocracks) in the coating, and the thickness of the deposited coating layer(s). All of these variables have a direct impact on coating costs and coating performance, thus presenting the designer with a variety of trade-off considerations.
U.S. Pat. No. 5,073,433 to Taylor discloses a thermal barrier coating. A thermal barrier coating is designed to protect turbine blades from thermal strain attributable to temperature cycles in the engine. The Taylor '433 thermal barrier coating is prepared from yttria (6-10 wt %) and zirconia powder having a mean particle diameter of about 40 microns. The coating is applied using a plasma spray process so as to intentionally generate between 20 and 200 vertical macrocracks per linear inch of the coating.
The coating of the Taylor '433 patent is applied by a complex process requiring repeated deposition (and cooling) of monolayers. This application process intentionally produces homogeneously distributed macrocracks throughout the coating. Construction of such a coating is time-consuming and dependent upon close control of process parameters. Abrasion resistance is not disclosed as a function of the Taylor '433 coating.
U.S. Pat. No. 5,520,516, also to Taylor, discloses a yttria-stabilized zirconia coating for turbine blade tips that provides abrasion resistance to the blade tips. This coating is applied to a turbine blade tip in a manner identical to that disclosed in the Taylor '433 patent, that is, by a complex process of monolayer deposition (and cooling) with the intent of creating at least 5 vertical macrocracks per linear centimeter of the coating. A post-deposition vacuum heat treatment is also recommended. The Taylor '516 patent further teaches that the applied coating be configured with a prescribed coating thicknesses at the edge of the blade tip to prevent shear adhesion failure of the coating on the blade tip.
U.S. Pat. No. 4,457,948 to Ruckle, et al. discloses a thermal barrier coating by plasma spraying. After deposition of a bond coat and a ytrria-stabilized zirconium ceramic coating, the resulting coating is heated to a temperature in the range of 820 centigrade to 1150 centigrade, then rapidly quenched (in, for example, liquid tin at 220 centigrade). The quenching operation produces an irregular pattern of cracks throughout the coating creating a columnar, segmented structure and a segmented, cracked coating surface.
Because of the inherent tendency of ceramic coatings to crack and the failure to withstand mechanical stresses, yttria-stabilized coatings have found wider application as thermal barrier coatings. Indeed, such careful preparation and prescribed edge thicknesses are necessary in the prior art use of yttria-stabilized zirconia for abrasive blade tip coating applications.
The present invention overcomes deficiencies in ceramic coatings, and in particular, abrasive blade tip coatings known to date. These deficiencies include the substantial expense required to apply these coatings in order to achieve the desired physical and mechanical properties. The coating of the present invention provides improved abrasion resistance with the attendant advantage of being applied in a simple application scheme. The present invention achieves a strong coating--substrate bond resistant to abrasive shear forces. The coating of the present invention exhibits a high tensile bond strength and very high lap shear strength, characteristics essential to a rub tolerant protective coating.